"16 studies have shown no causal association between vaccines and autism, and these studies carry weight in the scientific industry."-- Dr. Nancy Snyderman, NBC Today Show Medical Editor

Conventional wisdom holds that the autism-vaccine question has been "asked and answered," and that at least 16 large, well-constructed epidemiological studies have thoroughly addressed and debunked any hypothesis that childhood vaccination is in any way associated with an increased risk for autism spectrum disorders.

But there are several critical flaws in such an oversimplified generalization, and they are rarely given close examination by public health experts or members of the media.

To begin with, it is unscientific and perilously misleading for anyone to assert that "vaccines and autism" have been studied and that no link has been found. That's because the 16 or so studies constantly cited by critics of the hypothesis have examined just one vaccine and one vaccine ingredient.

And, the population studies themselves have had critical design flaws and limitations.

The current US childhood immunization schedule calls for 28 injections with 11 different vaccines against 15 different diseases by two years of age. Of those 11 vaccines, only the Measles-Mumps-Rubella (MMR) shot has been studied in association with autism, (although a CDC study of an MMR-plus-chickenpox vaccine did show that the risk for febrile seizures in infants was doubled.) Meanwhile, those 11 vaccines contain scores of ingredients, only one of which, thimerosal, has ever been tested in association with autism.

It is illogical to exonerate all vaccines, all vaccine ingredients, and the total US vaccine program as a whole, based solely on a handful of epidemiological studies of just one vaccine and one vaccine ingredient. It is akin to claiming that every form of animal protein is beneficial to people, when all you have studied is fish.

Now, a new study has shown that giving Hepatitis B vaccine to newborn baby boys may triple the risk of developing an autism spectrum disorder.

An abstract of the study was published in the September, 2009 issue of the respected journal Annals of Epidemiology. In it, Carolyn Gallagher and Melody Goodman of the Graduate Program in Public Health at Stony Brook University Medical Center, NY, wrote that, "Boys who received the hepatitis B vaccine during the first month of life had 2.94 greater odds for ASD compared to later- or unvaccinated boys."

The conclusion states that: "Findings suggest that U.S. male neonates vaccinated with hepatitis B vaccine had a 3-fold greater risk of ASD; risk was greatest for non-white boys." The authors used U.S. probability samples obtained from National Health Interview Survey 1997-2002 datasets.

Critics will point out that this sample was limited to boys born before 1999, so the results are only applicable to that U.S. male birth cohort, and that the study's cross-sectional design limits inferences on causality. Another weakness is that the autism diagnoses were parent reported.

On the other hand, these results are generalizable to US boys age 3-17 born prior to 1999; vaccination status was confirmed through medical records; and there was controlling for confounders that may be associated with care seeking behaviors. (The P-value equaled 0.032) The full manuscript is currently under review by another journal.

Assuming that the full manuscript is published in a peer-reviewed journal, it will be among the first university-based population studies to suggest an association between a vaccine and an increased risk for autism. And that would be in direct contradiction to all those MMR and thimerosal studies that purportedly found no such link.

Does that mean that Hepatitis B vaccine causes autism? Of course not (though any relative risk above 2.0 is generally considered to prove causation in a US court of law).

But there are other studies, both published and greatly anticipated, which might support a hypothesized causal association between HepB vaccine and ASD, at least in boys.

Any day now, data culled from CDC's Autism and Developmental Disabilities Monitoring network (ADDM), is expected to be published in the Morbidity and Mortality Weekly Report, and the numbers are expected to put the rate of autism at around 1 in 100, or higher.

ADDM researchers examine the education and (when possible) medical records of all eight-year-old children in selected US cities and states. They look only at eight-year-old cohorts to allow time for all diagnoses to be made, reported and counted.

So far, ADDM has published data from just two birth cohorts: children born in 1992 (eight-year-olds in 2000) and those born in 1994 (eight-year-olds in 2002). The 1992 cohort revealed an estimated ASD rate of one in 166, or 60-per-10,000. (This has since been revised to 67-per-10,000, or one in 150).

For the 1994 cohort, the estimate was virtually unchanged, at 66-per-10,000.

But now that number is expected to exceed 100-per-10,000 for the 1996 birth cohort, born just two years later. The overarching question, of course, will be, "why?"

There are many possible explanations, though a 50% increase in just two years is astonishing, no matter what its cause.

One possible answer is the Hepatitis B vaccine, (which also contained 25 micrograms of mercury containing thimerosal up until 2002). Introduced in 1991, it was the first vaccine ever given on a population basis to newborn babies (within the first three hours after delivery) in human history.

But according to the CDC's National Immunization Survey, only 8% of infant children received the Hep B vaccine in 1992, when that birth cohort showed an ASD rate of 1-in-150.

By 1994, the number of children receiving Hep B vaccine had reached just 27% -- and the cohort showed a similar ASD rate, though it did go up by as much as 10% in some locations between the two cohorts.

But by 1996, Hep B coverage rate had risen to 82%. And that is the cohort whose ASD rate rose to around 100-per-10,000 or more.

Correlation, obviously, does not equal causation. But the uptake rate of that particular immunization is at least one environmental factor that did demonstrably change during the period in question.

In addition, some recent studies and Vaccine Court decisions have supported the contention that Hepatitis B vaccine can damage myelin -- the nervous system's main insulating component -- at least in certain genetically susceptible adults and infants.

A study published last October in the journal Neurology found that children who received the Hepatitis B vaccine series were 50% more likely to develop "central nervous system inflammatory demyelination" than children who did not receive the vaccine.

Most of this increase was due to the Engerix B brand of the vaccine, manufactured by the UK's GlaxoSmithKline. That brand increased the risk of demyelination by 74%, and patients with confirmed multiple sclerosis were nearly three times more likely to develop the disorder.